1. Field of the Invention
The present invention relates to a U-shaped sheet pile with a low resistance to pile-driving.
2. Description of Related Art
Over the last eighty years, several million tonnes of U-shaped sheet piles have been used worldwide for the construction of supporting walls, for example during work connected with excavation, and with the building of dams, dykes and reservoirs.
A U-shaped sheet pile has a flat back (called the flange of the sheet pile) to which are connected two legs (called webs of the sheet pile) carrying interlocking elements so that the sheet pile has a plane of symmetry perpendicular to the back. To form a supporting wall, these U-shaped sheet piles are assembled using the interlocking elements, with their backs alternately located on either side of the plane passing through the central axes of the interlocking elements. This plane then forms the neutral bending plane of the U-shaped sheet pile wall.
The standard methods of driving the sheet piles into the ground are ramming and vibration. It is known that pile-driving operations require the development of a considerable amount of energy, which is proportional to the resistance of the sheet pile to pile-driving. For a given pile-driving method, this resistance to pile-driving is mainly a function of the soil characteristics and the transverse cross-section of the sheet pile.
The "height" or "depth" of a U-shaped sheet is defined as the distance between a plane passing through the central axes of the two interlocking elements and the outer face of the flange, and the "useful width" of a U-shaped sheet pile is defined as the distance separating the central axes of the two interlocking elements. Sheet piles with a large useful width in principle make it possible to reduce the operational costs, since fewer sheet piles need to be driven into the ground to produce a given length of wall. Deep sheet piles can have lower thickness of material at the level of the flange and the webs while providing a high section modulus, which of course reduces the cost price of the sheet piles. Hence the interest in using wide and deep U-shaped sheet piles with lower thickness of material at the level of the flange and the webs.
Today, U-shaped sheet piles, available on the market as standard sections, have useful widths from 400 to 600 mm and a "depth/useful width" ratio from 0.18 to 0.54. The commonest U-shaped sheet piles have a "depth/useful width" ratio greater than or equal to 0.25, or even greater than 0.30. The thickness of the flange lies between 7 and 20 mm, and the thickness of the webs between 6 and 12 mm.
However, it should be pointed out that wide and deep sheet piles with low thickness of material at the level of the flange and the webs also become rapidly unstable under difficult pile-driving conditions. Hence the importance of limiting the stresses to which these sheet piles are exposed during the pile-driving, i.e. of having sheet piles with as low a resistance to pile-driving as possible. Now, although the reduction in the thickness of material at the level of the flange and the webs undoubtedly has a beneficial effect on the resistance to pile-driving, it is observed that an increase in the "depth/useful width" ratio unfortunately has a very adverse effect on the resistance to pile-driving of U-shaped sheet piles.
That being the case, it will be appreciated that the present invention has found a solution which makes it possible to have a reduction in the resistance of a U-shaped sheet pile to pile-driving while improving the stability of the sheet pile when it is being used.
Belgian patent No. 433704, which was published in 1939, describes sheet piles having the form of an angle-iron. It teaches to reinforce the concave corner defined by the two webs of the sheet pile by an extra thickness of material.
U.S. Pat. No. 1,012,124 discloses very compact sheet piles with a web made on the principle of a flat arch. These arched sheet-piles are supposed to replace flat sheet piles. Their depth/useful width ratio of these sheet piles is less than 0.10. They are conceived to enable the construction of very thin walls, having a total wall thickness that is substantially equal the thickness of the interlocks, so that the thickness of the wall will be no greater than that of flat sheet piles. In a preferred embodiment the arched sheet pile has a local extra thickness of material at the convex side of its two corners. It is specified that the addition of metal at these points increases the inertia and modulus and therefore greatly strengthens the individual section and the completed wall. It is further specified that this extra material also increases the length of the bearing for interior bracing timbers and at the same time tends to prevent deformation of the arch when under pressure.